Picture your childhood tree house. Now, scratch the rope ladder and add a fireplace, kitchenette and living roof. Perched 20 feet above street level in South Lake Union, the Island Treehouse is a grown-up version of that childhood hideout: a unique 235-square-foot communal space at Sitka Apartments(1255 Harrison St.; 206.466.2432), a 384-unit complex that welcomed its first residents last November. The rustic, wilderness-inspired tree house structure, decked in wood paneling and accessible via a skybridge, was designed by Daniel Mihalyo of Seattle’s Lead Pencil Studio, who says it’s the first community tree house in an apartment complex. Unfortunately, it’s only open to residents and their guests—no interlopers allowed.